This invention relates to planar antenna systems and, more particularly, to such a system for providing angularly adjustable linearly polarized beams.
Antennas for use in satellite communications systems are generally required to operate over two frequency bands--one for the uplink (i.e., transmitting to the satellite) and the other for the downlink (i.e., receiving from the satellite). Further, the uplink (or transmit) and downlink (or receive) beams have different polarizations. Any antenna used for satellite communications must incorporate these features, in addition to providing adequate signal strength and the proper radiation pattern (beamwidth, sidelobes, etc.) for distinguishing between satellites. It is therefore an object of the present invention to incorporate both uplink and downlink features in a single, low cost, lightweight, antenna.
In a particular application, the transmit and receive beams have orthogonally directed linear polarizations. It is therefore another object of this invention to provide an antenna which accommodates such polarizations.
In this application, the polarization vectors of the satellite antenna and the satellite communications system antenna are each oriented in a respective single direction for the uplink and for the downlink and must coincide at all times in order for the two to communicate effectively. The apparent orientation of the satellite antenna's polarization will depend on its position relative to the satellite communications system antenna. If the latter antenna is required to communicate with a number of satellites in different locations, its polarization must be adjustable. In this particular context, adjustable means rotatable about an axis orthogonal to the linear polarization vector. In addition, the uplink and downlink polarizations must be kept orthogonal, which implies that both must be rotatable. Further, if the satellite communications system antenna is mounted to a moving body, such as an aircraft, its polarization must be rotatable on a real-time basis to take account of the changing position of the antenna. It is therefore a further object of this invention to provide a linearly polarized antenna wherein the direction of polarization is rotatable (i.e., angularly adjustable) on a real-time basis.